Depends on the trailride, if its a day trip or an over nite. But the essentials are water, snacks, rope halter, compass, first aid kit, about 25 ft of paracord, bug spray, and rain gear. I have a sharp knife in my pocket, a pistol on my belt, and just in case the unexpected happens, and I get stuck out all night (it has happened) in my shirt pocket I carry a an old altoids tin, with a lighter and Vaseline soaked cotton balls so if I have to start a fire its easy. I also keep a small Ziploc baggie with some instant coffee, an old tin cup from my boy scout days and hard candy to get me thru the night.
I realize some of it might seem superfluous but a group of us got stuck out all night, after missing a turnoff. Its embarrassing we were all experienced riders, and familiar with the area, we just rode right past it! We spent a miserable night in the skeeters, with no fire, and nothing to pass the time with, and then it rained! However it was a heckuva learning experience.